FRS 6. ANATOMY OF THE SMALL AND LARGE INTESTINES Flashcards
What are the sections of the large intestine?
o caecum
o appendix
o ascending colon, transverse colon, descending colon
o sigmoid colon (then rectum and anal canal)
What are the characteristic features of the large intestine?
Appendices epiploicae – tags of fat (small pouches of peritoneum,
filled with fat) attached to the surface of the large intestine
Teniae coli – the outer longitudinal muscle is arranged into three
bands of muscle
Haustra – when the teniae coli contract, the walls of the bowel
shorten to form sacculations known as haustra
What is the ileocaecal junction
The large intestine joins the small intestine at the ileocaecal junction
o demarcates the caecum from the ascending colon
What is the appendix and where is it located?
also known as the vermiform (worm-shaped) appendix is a narrow, blind ended tube
attached to the posteromedial end of the caecum.
o The position of the free-end of the appendix is highly
variable, and can be categorised into seven main
locations.
o The most common positions are retrocaecal and subileal
o The appendix can be located by looking at the point at
which the 3 bands of taenia coli converge
What is the caeum?
Most proximal part of the large intestine – located between the
ileum and the ascending colon
Used to serve as a site for cellulose digestion in ancestors but no
real function now apart from acting as a reservoir for chyme that it receives from the ileum
Describe the ascending colon
retroperitoneal; ascends superiorly from the caecum.
When it meets the right lobe of the liver, it turns 90 degrees to move horizontally
This turn is known as the right colic flexure (or hepatic flexure); marks the start of the transverse colon.
Describe the transverse colon
crosses the abdomen, extends from the right colic flexure to spleen;
turns 90 degrees to point inferiorly.
This turn is known as the left colic flexure (or splenic flexure).
Here, the colon is attached to the diaphragm by the phrenicocolic
ligament.
The transverse colon is the least fixed part of the colon, and is variable
in position
Unlike the ascending and descending colons, the transverse colon is
intraperitoneal
Describe the descending colon
After the left colic flexure, the colon moves inferiorly, and so is termed the descending colon
retroperitoneal in most people, but anterior to the left kidney, passing over its lateral border
When the colon begins to turn medially, it becomes the sigmoid colon
Describe the sigmoid colon
40cm long; located in theleft lower quadrantof the abdomen, extends from left iliac fossa to level of S3
vertebra
characteristic “S” shape
The sigmoid colon is attached to the posterior pelvic wall by a mesentery – thesigmoidmesocolon
The long length of the mesentery permitsthis part of the colon to be particularly mobile.
What are the peritoneal folds
Only two parts of the colon are supported by peritoneal folds known as the mesentery
o Transverse mesocolon => this structure attaches the colon to the back abdominal wall
o Sigmoid mesocolon
These carry blood and lymphatic vessels
What are the paracolic gutters?
two recesses between the ascending/descending colon and the posterolateral abdominal wall
clinically important – allow infective material that has been released from the abdominal organs to
accumulate
What are the sections of the small intestine?
- duodenum
- jejunum
- ileum
What is the duodenum and what are its sections?
Most proximal portion of the small intestine – it is C-shaped and
adjacent to the pancreas (retroperitoneal)
Divided into four parts which collectively form C-shape:
- superior
- descending
- inferior
- ascending
Describe the superior section of the duodenum
SUPERIOR (L1)
5cm in length
Ascends upwards from pylorus of the stomach
Connected to liver by the hepatoduodenal ligament
The initial 3cm is covered by the visceral peritoneum remainder is
retroperitoneal
Describe the descending section of the duodenum
DESCENDING (L1-3) 7.5 cm Curves around the head of pancreas – lies posterior to transverse colon and anterior to right kidney Contains the major duodenal papilla
Describe the inferior section of the duodenum
10 cm
Travels laterally to the left
Crosses over the IVC and AA, located inferior to the pancreas
Describe the ascending section of the duodenum
2.5 cm
After crossing aorta it ascends and curves anteriorly to join the jejunum at a sharp turn known as the
duodenojejunal flexure
located at the duodenojejunal junction is a slip of muscle called the suspensory muscle of the duodenum
contraction widens the angle of the flexure, and aids movement of the intestinal contents into the
jejunum
Describe the jejunum
Represents two fifths of the last two sections of the small intestine (40%)
Begins at the duodenojejunal flexure
Mostly located in the left upper quadrant
larger diameter and thicker wall than ileum
Inner mucosal lining of the jejunum characterised by presence of plicae
circulares (circular folds) => most pronounced in the jejunum
Describe the ileum
Makes up the distal three-fifths of the small intestine (60%)
Mostly found in the right lower quadrant
Has thinner walls and fewer, less prominent plicae circulares
Has more mesenteric fat and arterial supply
Opens into the large intestine – where caecum and ascending colon join together
Two flaps project into the lumen of cecum – called the ileocecal fold flaps
o The folds come together forming muscular ridges – acts as a sphincter to prevent reflux of
material
Which sections of the small intestine are intraperitoneal?
both the jejunum and ileum are intraperitoneal as they are both attached to the posterior abdominal wall by
mesentery (a double layer of peritoneum)both the jejunum and ileum are intraperitoneal as they are both attached to the posterior abdominal wall by mesentery (a double layer of peritoneum)
Briefly describe the blood supply to the gut
ascending colon + proximal 2/3 rds of the transverse colon + jejunum and ileum
o derived from the midgut => supplied by the superior mesenteric artery
distal 1/3 rd of transverse colon + descending colon + sigmoid colon
o derived from the hindgut => these structures are supplied by the inferior mesenteric artery
Describe the blood supply to the jejunum and ileum
Superior mesenteric artery arises from the abdominal aorta, it then gives
off branches to the left supplying the jejunum (located in the upper left
part of the infracolic compartment)
o There are connections formed between the small branches
known as ARTERIAL ARCADES
There are more branches given off from the superior mesenteric artery
to the ileum there are MORE ARTERIAL ARCADES in the ileal branches
From the arcades, long and straight arteries arise, called vasa recta
jejunum has longer vasa recta and the ileum has shorter vasa recta
The blood vessels run through the mesentery
Describe the blood supply to the ascending colon and proximal 2/3 of the transverse colon
The superior mesenteric artery also gives rise to blood vessels on the right – a total of three branches of
colic arteries supplying different areas of the colon
o Each colic artery has two branches
MIDDLE COLIC ARTERY – supplies proximal 2/3 of the transverse colon
RIGHT COLIC ARTERY - goes towards the hepatic flexure and supplies the ascending colon
ILEOCOLIC ARTERY - supplies last part of ileum, caecum, and appendix
NB: The middle, right and ileocecal branches anastamose to form the MARGINAL ARTERY; the marginal artery
itself gives of small branches through its length
Describe the blood supply to the duodenum
Proximal major duodenal papilla– gastroduodenal artery(branch of thecoeliac trunk).
Distal to major duodenal papilla– inferior pancreaticoduodenal artery(branch ofsuperior mesenteric
artery).
Describe the blood supply to the hindgut structures
descending colon - left colic artery (branch of the inferior mesenteric artery)
sigmoid colon- sigmoid arteries (branches of the inferior mesenteric artery)
o become the rectal artery
What is the lymphatic drainage of the small intestine?
Duodenum: pancreatoduodenal and superior mesenteric nodes.
Jejunum and Ileum: superior mesenteric nodes.
Describe the lymphatic drainage of the colon
ascending and transverse colon - superior mesenteric nodes.
descending colon and sigmoid - inferior mesenteric nodes
Most of the lymph from the superiormesenteric and inferior mesenteric nodes passes into theintestinal
lymph trunks, and on tothecisterna chyli–where it ultimately emptiesinto the thoracic duct.
What is meckel’s diverticulum
EMBRYOLOGICAL ABNORMALITIES
AMeckel’s diverticulum, a truecongenital diverticulum, is a slight bulge in thesmall intestine present at birth and avestigial remnant of the omphalomesenteric duct(also called the vitelline duct or yolk stalk).
most common malformation of thegastrointestinal tract and is present in approximately 2% of the population,with males more frequently experiencing symptoms.
Describe the histology of the duodenum
The gastroduodenal junction which is at the pyloric sphincter, marks the sharp transition from the
glandular mucosa of the stomach to the villous mucosa of the duodenum
o Note the thickening of muscle at the pyloric sphincter
The duodenum is distinguished from jejunum and ileum by the presence of brunner’s glands
(predominantly found in the submucosa – poorly stained)
o secrete an alkaline fluid composed of mucin; exerts a physiologic anti-acid function by coating
the duodenal epithelium, protecting it from the acid chyme of the stomach
o also produce lysozyme and EGF
What are the 3 stages of folding in the small intestine
- plicae circulares
- villi
- microvilli
Describe plicae circulares
large circular folds with a submucosal core
o Muscularis externa lies beneath it – supporting the villi
Describe the villi
cover the plicae circulares
o Interspersed with short glands known as crypts of Lieberkühn which extend down to the
muscularis mucosa (project into the lamina propria)
o Villi may contain:
Blood capillaries
lacteals
strands of smooth muscle (from muscularis mucosae) gives villi contractile element
This may help squeeze the lymph along lacteals
o NB: the base of villi represents the “true surface” of the small intestinal epithelium
Describe the microvilli
found on top of the enterocytes – form the brush border
What are Peyer’s patches?
lymphoid aggregates found in submucosa
What is the lining of the intestinal villi?
intestinal villi are lined by simple columnar epithelium which is continuous with that in the crypts
The epithelium of the villi is made up of tall columnar absorptive cells calledenterocytes, andgoblet cells,
Lamina propria extends between crypts and into core of each villus – rich vascular and lymphatic network
Describe enterocytes
o Most numerous cell type
o Tall columnar cells with microvilli – which forms the brush border (increased surface area)
Brush border formed by microvilli
The goblet cells are poorly stained
Microvilli are surrounded by a glycocalyx
o absorptive cells
o There are tight junctions between enterocytes
What are the additional cells found in the crypts of Lieberkühn?
Paneth cells:
o Found at the base of the crypts; distinguished by their eosinophilic granules (granular cytoplasm)
o Secrete granules of antimicrobial peptides called defensins, lysozyme and phospholipase A
Endocrine cells:
o Produce secretin, somatostatin, enteroglucagon and serotonin
Stem cells:
o Found at base of crypts – replace enterocytes, goblet cells, paneth cells etc.
Describe the histological differences throughout the small intestine
- The villi tend to be longest in the duodenum and shortest in the ileum
- Lymphoid tissues become more prominent in the ileum (very sparse in the duodenum)
- Proportion of Goblet cells in epithelium increases distally
- Plicae circulares are most prominent and numerous in the jejunum – not as prominent in the
proximal duodenum and distal ileum
Describe the histology of the large intestine
The thickmucosahas deep crypts, but there are no villi
Epithelium = columnar absorptive cells with a striated border, many goblet cells, endocrine cells and
basal stem cells, but no Paneth cells
The surface epithelial cells are sloughed into the lumen, and have to be replaced around every 6 days.
Thelamina propriaandsubmucosaare similar to the small intestine
Thelongitudinal smooth musclein themuscularis externais arranged in three longitudinal bands
calledtaenia coli
At the anus, the circular muscle forms the internalanal sphincter.
Describe the histology of the appendix
Small, star-shaped lumen
Infiltrated with lymphocytes
arrangement of the layers in its walls is similar to that seen elsewhere in the large intestine
However, the outer layer of muscle fibres in themuscularis externaforms a continuous layer.
presence of masses oflymphoid tissuein themucosaandsubmucosa
There are often follicles containing paler germinal centres, similar to the follicles of Peyers patches
